Generally, dashes are used in text to indicate a break in thought or sentence structure, to separate two clauses, or to introduce a phrase added for emphasis, definition, or explanation. They can be used singly or in pairs. British English commonly uses an en dash (–), while the longer em dash (—) is the standard dash in American English. As dashes can get lost on screen, the best dash to use is the longest, the em dash, in general onscreen text. It should have no spaces between it and the letters beside it.
The dog—having been bred in the country—was uneasy in the city.
I was amazed to see her there—she never goes out.
Do not capitalize the word following an em dash unless it is a proper noun, even if the text following the em dash ...